A charged particle optical system is a system for focusing, dispersing, spectral-dispersing, deflecting, transporting, or treating otherwise, charged particles such as an ion or an electron by the effect of an electric field or magnetic field. In the following explanation, an ion optical system in which an ion is particularly exampled as a charged particle is described. However, the same is basically true for other charged particles such as an electron.
As an ion optical system for deflecting an ion beam or separating ions in accordance with the mass or energy, a sector-shaped electric field which has a simple configuration and good versatility is often used. Examples of a sector-shaped electric field include a multi-turn time-of-flight mass spectrometer as described in Patent Document 1. In a multi-turn time-of-flight mass spectrometer, a plurality of sector-shaped electric fields are used to form an ion orbit of a closed orbit such as a substantially elliptical orbit, substantially “8” figured orbit, etc. Ions are made to fly along this ion orbit a number of times so that an effectively long flight distance is ensured in order to enhance the mass separation performance on ions.
In such a multi-turn time-of-flight mass spectrometer, an ion source for generating ions and an ion detector for detecting ions may be provided on the orbit in some cases. However, in many cases, ions generated outside the orbit are introduced into the orbit and made to fly around on the orbit for predetermined times, and then the ions are deviated from the orbit and introduced into an ion detector provided outside the orbit to be detected. In order to introduce ions into an orbit and take out ions from the orbit as previously described, it may be required to provide an opening in a sector-shaped electrode for forming a sector-shaped electric field (refer to aforementioned Patent Document 1). In addition to the entrance/exit of ions into or from the orbit, it may be required to make an opening in the sector-shaped electrode to introduce electromagnetic wave such as laser light or X-ray, or a particle beam from the outside in order to monitor the status and mode of the ions flying around on the orbit.
However, in the case where an opening is provided in the sector-shaped electrode as previously described, it is inevitable that the electric field is disturbed near the opening. With the disturbance in the sector-shaped electric field, the orbit of ions becomes nonideal (or not as designed) when they pass through the area, which makes the problems pronounced such as: the mass accuracy is decreased particularly in going around many times or the pass-through ratio of ions is deteriorated and consequently the detection sensitivity is decreased.
Conventionally, in order to alleviate the effect of the disturbance in the electric field due to the provision of an opening in a sector-shaped electrode, a metal mesh, metal wires, or other element is placed at the opening. However, since such a mesh and wire also pose impediment to the passage of ions, they lead to the deterioration of the pass-through ratio of ions and the deterioration of the pass-through efficiency of electromagnetic wave such as laser light or particle beam. Furthermore, since the effect of the disturbance in the electric field remains even with a metal mesh and wire, using such an element is not an appropriate method particularly in the case where an accurate analysis is required.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H11-135060